1. Field of the Invention
Preferred aspects of the present invention relate to a device that creates Taylor vortices on at least one side of a filter, thereby improving mass transfer and minimizing concentration polarization. Preferred embodiments of the present invention are particularly useful in dialysis of blood from patients with kidney disease. In other embodiments, the present invention can be used in areas of heat and mass transfer.
2. Description of the Related Art
Traditionally, dialysis is the maintenance therapy used to treat kidney disease. There are two common approaches. One is peritoneal dialysis, where the process is done internally to the patient, in the patient's pericardium. Peritoneal dialysis uses the patient's abdominal lining as a blood filter. The abdominal cavity is filled with dialysate, thereby creating a concentration gradient between the bloodstream and the dialysate. Toxins diffuse from the patient's blood stream into the dialysate, which must be exchanged periodically with fresh dialysate.
The second approach is by filtration dialysis. This was initially accomplished using flat sheet dialysis membranes, requiring square meters of the membranes. Devices were large and taxing on patients. In the 1960's, hollow fiber dialysis filtration units became popular. This was an improvement, as a large filter membrane area could be compressed into a small volume, and the volume of blood needed to fill the unit was greatly reduced.
While hollow fiber technology provides a relatively safe and cost effective means for dialysis, problems remain. Manufacturing hollow fiber cartridges is challenging. The patient is still exposed to a large surface area of material foreign to the human system. Many of the chemicals needed in manufacture are toxic to the patient. Cuprophane is the most common membrane material for hollow fiber manufacture, but it has biocompatibility issues, and relatively low permeability performance. There are superior membrane materials available in flat sheet, but these materials are challenging to form into hollow fibers.
One of the most limiting problems in any type of filtration process, including dialysis, is filter clogging, scientifically described as “concentration polarization.” As a result of the selective permeability properties of the membrane, the filtered material that cannot pass through the membrane becomes concentrated on the surface of the membrane. This phenomenon is clearly illustrated in the case of a “dead-end” filter, such as a coffee filter. During the course of the filtration process, the filtered material (coffee grounds) building up on the filter creates flow resistance to the filtrate, the fluid (coffee), which can pass through the filter. Consequently, filtrate flux is reduced and filtration performance diminishes.
Various solutions to the problem of concentration polarization have been suggested. These include: increasing the fluid velocity and/or pressure (see e.g., Merin et al., (1980) J. Food Proc. Pres. 4(3):183-198); creating turbulence in the feed channels (Blatt et al., Membrane Science and Technology, Plenum Press, New York, 1970, pp. 47-97); pulsing the feed flow over the filter (Kennedy et al., (1974) Chem. Eng. Sci. 29:1927-1931); designing flow paths to create tangential flow and/or Dean vortices (Chung et al., (1993) J. Memb. Sci. 81:151-162); and using rotating filtration to create Taylor vortices (see e.g., Lee and Lueptow (2001) J. Memb. Sci. 192:129-143 and U.S. Pat. Nos. 5,194,145, 4,675,106, 4,753,729, 4,816,151, 5,034,135, 4,740,331, 4,670,176, and 5,738,792, all of which are incorporated herein in their entirety by reference thereto). In U.S. Pat. No. 5,034,135, Fischel discloses creating Taylor vorticity to facilitate blood fractionation. Fischel also describes variations in the width of the gap between a rotary spinner and a cylindrical housing, but does not teach variation in this width about a circumferential cross-section.
Taylor vortices may be induced in the gap between coaxially arranged cylindrical members by rotating the inner member relative to the outer member. Taylor-Couette filtration devices generate strong vorticity as a result of centrifugal flow instability (“Taylor instability”), which serves to mix the filtered material concentrated along the filter back into the fluid to be processed. Typically, a cylindrical filter is rotated within a stationary outer housing. It has been observed that membrane fouling due to concentration polarization is very slow compared to dead-end or tangential filtration. Indeed, filtration performance may be improved by approximately one hundred fold.
The use of Taylor vortices in rotating filtration devices has been applied to separation of plasma from whole blood (See e.g., U.S. Pat. No. 5,034,135). For this application, the separator had to be inexpensive and disposable for one-time patient use. Further, these separators only had to operate for relatively short periods of time (e.g., about 45 minutes). Moreover, the separator was sized to accept the flow rate of blood that could reliably be collected from a donor (e.g., about 100 ml/minute). This technology provided a significant improvement to the blood processing industry. The advantages and improved filtration performance seen with rotating filtration systems (Taylor vortices) have not been explored in other areas of commercial fluid separation—including kidney dialysis.
The use of Taylor vortices does not alleviate all problems with filtration however. Another common problem with the use of such rotating filtration devices is concentration polarization on the inner side of the filter membrane. While centrifugal flow instability circulates the fluid between inner and outer members, the rotating inner member does not prevent concentration polarization near the walls of its interior. As a result, filter performance could be further improved by solving this problem of interior concentration polarization.